Generally, thermal recording materials having a thermosensitive color developing layer based mainly on a colorless or pale-colored electron-donating leuco dye (hereinafter also simply referred to as “dye”) and an electron-accepting developer (hereinafter also simply referred to as “developer”) that reacts with the dye to develop a color upon heating are widely used in practical applications. Image (information) recording on such a thermal recording material is usually achieved by heating a portion to be imaged on the thermosensitive color developing layer using a thermal head. This method of thermal recording is advantageous over other conventionally known methods of recording, with features such as noiselessness during recording, obviation of the need for development and fixation, freedom from maintenance work, relatively inexpensive instrumentation, compactness, and very vivid colors developing in the images obtained, and is widely employed for facsimiles, computer terminal printers, automated ticket machines, measurement recorders, handy terminals for outdoor use, and the like. Also, thermal recording materials are coming to be used not only for output paper for these various devices, but also for betting ticket paper and the like, which are required to have high storability.
When thermal recording materials are used for various tickets, receipts, labels, bank ATM output paper (sheets), gas, electricity and tap water meter reading output paper (sheets), bicycle race, horserace and other betting tickets, and the like, there is a demand for plasticizer resistance and oil resistance that ensure freedom from problems with print portion readability even during storage in contact with plastic films, synthetic leather and the like for a long time, and for color fastness to light and heat resistance that prevent recorded images from discoloring even when exposed to sunlight for a long time. Disclosed for this reason are thermal recording materials incorporating a particular diphenylsulfone derivative as a developer added to improve print portion storability in terms of plasticizer resistance, oil resistance, heat resistance and the like (Patent Documents 1 and 2).
[Document List]
[Patent Documents]
    patent document 1: JP-A-2003-212841    patent document 2: JP-A-H08-333329